High-purity metallic silicon having a resistivity of 0.5-1.5 Ω·cm or higher and a purity of 99.9999% (6 N) or higher is generally used for polysilicon solar cells. The most desirable industrial process for producing this high-purity metallic silicon is a process in which raw-material metallic silicon that contains impurities in a large amount and has a low unit price is refined to remove impurities therefrom and thereby produce the high-purity metallic silicon.
Among the impurities contained in raw-material metallic silicon, iron, aluminum, and calcium can be removed by subjecting the molten silicon to solidification/segregation to thereby cause these impurity metals to remain in the liquid phase of the molten silicon. Calcium and the like can be removed by subjecting the molten silicon to a vaporization treatment in a vacuum of about 1.3×10−2 to 10−4 Pa (10−4 to 10−6 Torr), although this method requires much time.
However, boron and phosphorus, among the impurities, are exceedingly difficult to remove. Removal of boron is especially difficult. For example, an oxidation treatment is being conducted in which the boron is gasified and removed as a compound thereof with oxygen or hydrogen, for example, by adding oxygen, carbon dioxide, or water vapor to argon as an inert gas and bubbling this mixed gas into the molten silicon (patent document 1 and patent document 2).
The method described above has drawbacks that the operation for oxidizing the boron (B) contained in raw-material metallic silicon using, for example, water vapor and removing the boron as BO gas requires much time, and that silicon oxidation simultaneously occurs, resulting in a large loss. Especially when water vapor is blown into the molten silicon, a side reaction occurs to evolve hydrogen in a large amount. There has hence been a problem concerning safety.
There also is a method in which silicon prepared by melting raw-material metallic silicon is subjected to a vaporization treatment in a vacuum of about 1.3×10−2 to 10−4 Pa (10−4 to 10−6 Torr) in order to remove phosphorus. However, this method has had a problem that the treatment requires much time and is costly because the treatment is a high-vacuum process. Namely, there has been a problem that the boron and phosphorus to be removed must be removed by respective separate processes which are costly.
Meanwhile, as a method for refining silicon using an alkali halide, a technique has been proposed in which slag is formed from sludge of raw-material metallic silicon (the slag includes, as a main component, silicon dioxide which was present in the raw-material metallic silicon) and the slag is used for compositional regulation during impurity removal to recover the silicon (patent document 3). However, silicon having an entirely satisfactory purity has not been obtained.
Furthermore, patent document 4 describes a step in which 20 g of a raw-material metallic silicon powder is pulverized and mixed, in a weight ratio of 1:1, with NaF having the same particle diameter as the silicon powder, a step in which the powder mixture is heated at 1,300° C. and the solid silicon is brought into contact with the molten NaF, a step in which a second sample is heated at 1,450° C. for 10 minutes to melt the NaF and the raw-material metallic silicon, a step in which these samples (NaF and silicon) are cooled to room temperature, and a step in which the silicon is separated from the NaF contained in each sample by extraction with an aqueous medium and by succeeding decantation and filtration.
However, the process described in patent document 4 is a mere technique for silicon refining in which silicon is separated from solid matter containing NaF and raw-material metallic silicon by means of filtration, etc. to thereby refine the silicon. The process has had problems that the refining effect is insufficient and that the operation for separating silicon is not easy.